Understanding Underlay and Overlay networks

[YumeNoKira]

Hello community,

First of all if that question has already been asked i'm apologizing in advance for i have not found it in the forum.

I'm beginning a new step of my career in networks and that takes me to DC architectures design. I am trying to catch up with all the notions that i lack (and that's a lot). Before digging into VXLANS, EVPN, ECMP, Nexus switches etc i realized that i could not make sense of the concepts of underlays and overlays.

I've seen some courses and they basically say stuff that are weird to me like "Underlay is responsible for delivery packets" and "Overlay transmits packets only along the virtual links between the overlay nodes". This may be because i'm not a native english speaker but to me both these statements are kinda contradictory.

May be the courses that i've found were not dumbed enough for me  

If anyone could help me understand these concepts (the differences and the added value of an overlay) i'd be grateful thank you !

[Sergiu.Daniluk]

Hi @YumeNoKira 

The easiest way to understand the concept of underlay/overlay is to visualize it. There are quite a lot of blogs/courses online with good drawings, but I just googled "vxlan overlay underlay" on google and I found this image:

Reference: https://networklessons.com/cisco/ccnp-encor-350-401/introduction-to-virtual-extensible-lan-vxlan

So basically the underlay is the actual physical topology doing the routing. While the overlay is the virtual network interconnecting the end devices between which overlay protocol is configured. 

If we look at the packet itself, this is how it looks in overlay (the blue portion - the original frame) vs underlay (the red+blue - the encapsulated frame):

Hope it helps,

Sergiu